1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a material for alignment layer of liquid crystal display (LCD).
2. The Related Arts
Liquid crystal used in twisted nematic (TN) or super twisted nematic (STN) liquid crystal display is positive liquid crystal. The liquid crystal molecules have a long axis that is arranged parallel to a substrate surface when no electricity is applied. The orientation of the liquid crystal molecules on the substrate surface is determined by rubbing direction of an alignment layer (which is usually made of polyimide). The alignment directions of two substrate surfaces are perpendicular to each other, whereby the molecules of the liquid crystal layer exhibit a continuously twisted arrangement from one substrate surface to another substrate surface. When a voltage is applied, the long axis of the liquid crystal molecule tends to align in the direction of electric field. Drawbacks of the TN or STN liquid crystal display are small viewable angle and severe brightness difference and color difference at large view angles. A compensation film must be applied to correct these problems and this increases the manufacture cost of display devices.
To overcome the various limitations and drawbacks of the TN/STN liquid crystal display, vertical alignment (VA) has been recently developed. Negative liquid crystal is used in VA technology, in which when no electrical field is applied, the liquid crystal molecules are arranged to perpendicular to substrate surfaces and when an electrical field is applied, the liquid crystal molecules are rotated or tilted toward a direction parallel to the substrate.
To overcome the limitation of view angle, the VA technology adopts various ways to effect multiple angle alignment. Namely, a pixel is divided into multiple domains and liquid crystal molecules in different domains are caused to tilt in different directions so that the display, when viewed from different directions, can provide similar viewing effect. Several ways discussed below are currently adopted to realize multiple angle alignment. As shown in FIG. 1A, the first way is to form polymer bumps 108 on upper and lower glass substrates 102, 103 of an LCD by means of exposure development in order to cause a pre-tilt angle for liquid crystal molecules around the polymer bumps 108 and thus guiding the liquid crystal molecules 106 to tilt in predetermined directions. As shown in FIG. 1B, the second way is to form an ITO pixel electrode 304 on upper and lower glass substrates 302, 303 in such a way as to form a predetermined pattern so that an electric field generated thereby shows a predetermined tilt angle thereby controlling the orientations of the liquid crystal molecules 306 in different domains. This technique is often referred to as patterned vertical alignment (PVA). As shown in FIG. 2, the third way is to form ITO slits 501 on a TFT side of a lower glass substrate 500, while ITO 504 of an upper glass substrate 502 remains intact. Polymerizable monomer 508 is added in the liquid crystal medium. An electric field is first applied to cause tilting of the liquid crystal molecules 506 and then ultraviolet ray is applied to irradiate the panel so as to polymerize the monomers to form polymer bumps 510 that guide the tilting of the liquid crystal molecules 506 and are deposited on the surfaces of the upper and lower glass substrates 500, 502 to achieve an effect of alignment. This technique is referred to as polymer stabilized vertical alignment (PSVA).
The above three ways of effecting alignment are more or less difficult and complicate in either the process of the material. Aperture ratio and contrast must be sacrificed in order to achieve multiple angle alignment and this deteriorates the optic performance of a liquid crystal panel.